In many industrial manufacturing processes. The process is often large and complex automated control system was used for the production of a continuous bloom. Performs well. Important one in the system that not only is a device driver or the final drive, which will be discussed in this case, control valve, which is one of many in the group's final drive, because the control valve is a device that is very important. One of the production system. Also known as a top priority there.
Education for understanding will lead to safer use of the system. And practitioners. As well as the production process more efficient and effective as desired. The problem with that fits the era when the power is often scarce and more every day.
Before going into the contents of the Control valve should know and understand the basics about Process Control Terminology in order to be able to understand the next section as well. It said in a brief summary. And goes into detail in the next section again. This case is very similar to the study of mathematics to understand fully before it can be defined in part by an interruption to a minimum.
It is recommended to keep reading until the end, you will be most useful.
Note: final drive = final control element = final element.
1. Process Control Terminology.
1.1 Capacity is the Flow Rate of flow of fluid through the Control Value under certain conditions.
1.2 Controller is a device that acts in the process. By accepting or Input variables to control the variables measured (Controlled Variable or Measured Variable), then processed as a control (Control Regulatory) and then send the Output Signal (The outcome of the process. effect) to the Final Control Element in the Control Value.
1.3 Closed Loop Control of a closed control circuit. The present value of the Status of Final Control Element back compared to the Set Point to determine the status of the target (Set Point) is also a Error or SP-PV of the Controller, it is ordered. Continue until the status of the Final Element required by the Controller Error as it will stop sometimes to the Closed Loop Control Loop the Loop called the special attention that we want to control.
1.4 Control Range is the range of motion of the Stem Value or range of motion of the stem valve sometimes called the valve travel or rated travel.
1.5 Dead Band or Dead Zone.
The sensor or control valve does not respond to the changes (input. The BOT conversion) as a result of the nature of the device itself. Occur in both instrument and control valve, but most will apply to each instrument and control system.
From below, described as follows.
Assume that this system is a function of the air compressor has a set point to start at 7.5 bar, stop at 8.0 bar.
At the start air compressor to compress the air to generate pressures up to 8 bar of pressure switch to tailor the air comp. But because the time lag was found to occur over shoot up a little later on. pressure in the system gradually decreases until it reaches 7.5 bar, where the compressor to start up, but because the pressure switch to use a dead band is quite large (equipment, low quality), making it impossible to respond to the input variable and air compressor will not work as intended because the switch does not change the status. Decreasing the pressure to 7.0bar of the air compressor to start up the dead band can be seen that this happens, we can not control the system as needed.
How is the pressure switch with low dead band to use or install a pressure switch 2 is made to start, stop independently of each other.
dead band of instrument.
The documents for the course introduction to process control training plan. Electricity Generating Authority of Thailand
1.6 Gain is the Output Change / Input Change has 2 types of Static Gain is the OP Change / IP Change during Steady State, sometimes called the Sensitivity of Dynamic Gain an OP Change / IP Change in conditions where the change occurs. as a Function of Input frequency or rate of change in this value is also equipped with a means to extend the input signal is enough. For example, the power amplifier.
1.7 Hysteresis as a result of reversing the dead band is not. A unique feature of the device (instrument), most of the material and the identification feature of the device can not be solved.Imagine a line drawn on paper from left to right. Then drag it back by trying to replace the old line that is not possible. This means that any movement in the world that happens, it can not reverse it completely. According to Rule 2 of the thermodynamics.
The Control valve handbook, third edition, FISHER Controls international LLC, USA.
1.8 Open Loop Control is an open circuit. Without bringing the status of Final Control Element back compared to the Set Point to determine if there is Error or SP-PV occurred such as opening a floodgate is ordered to open the door motor is running. until the end. By monitoring the position of the door where there is no sensor that detects movement in various positions there.
Working in this manner is not commonly used in automatic and very rarely found in automatic process control.
1.9 Process Variable, including pressure, temperature, flow rate, level, conductivity, p H, vibration etc.
1.10 Process or processing and oil refining process. Production of electricity. The cooling process.
1.11 Relay is a device that acts similar to the Power Amplifier is to amplify the air (pneumatic. signal) to a larger drive for Value Actuator word in this sense is used with a control valve.
The Control valve handbook, third edition, FISHER Controls international LLC, USA.
1.12 Resolution is the resolution of the Display device or the display. Can be expressed in value. This means that with only minor changes. It can be shown that as pressure gauge for pressure measurement with a scale model should assume that a channel of the scale is equal to 0.5 psi, it means that if the pressure changes of less than 0.5 psi, it can not. read correctly. It can be about value. The digital display is a set of numbers such as 2 is x.xx If the change is at 0.00x, it can not display it to appear. The meter will not recognize the change at all. Unlike analog or analog to estimate the change is simply not correct it.
1.13 Response Time or as often referred to as Time Constant.
The time response of the process to change the value of the Sensor input. The Response Time is typically measured in terms of the Time Constant is a value.
Time Constant Output refers to the value of which changes with time depending on the input and ideas from the Output to Output 63.2% of all the changes.
Response from a graph of the temperature. Normal temperature is 100 oF if the temperature was suddenly changed to a Step Change to 700 oF the response of the device will be gradually increased to 63.2% of the 600 oF (700-100).
From the graph it is seen that the results of the measurements of the Sensor and pointed to the 63.2% or 479.2 oF (600 * 63.2%), it takes 3 seconds, that is, 1 Time Constant is therefore concluded that 1 time constant is. equal to 3 seconds.
The other is calculated as follows: Time Constant.
2 Time Constant (6 seconds).
700 o F - 479.2 o F = 220.8 o F.
(220.8oF x 0.632) + 479.2 oF = 618.7 oF.
3 Time Constant (9 seconds).
700 oF - 618.7 oF = 81.3 oF.
(81.3 oF * 0.632) + 618.7 oF = 670.5 oF.
1.14 Shaft Wind-Up is the nature of the movement (rotation) of the strength of the Actuator Shaft on the bottom directly to the rotation. But the end is connected to the Value Plug the motion due to friction and Actuator Seal or packing far more torque to overcome friction until Value Plug the move. This only happens with Rotary Value.
The Control valve handbook, third edition, FISHER Controls international LLC, USA.
1.15 Sensor and Transmitter.
Sensor is a device parameter (Measured Variable) include a temperature sensor, pressure sensor, level sensor and so on.
Transmitter is a device which is used to change the values of the variables measured (Measured Variable) is to get value from the sensor it can then convert to a standard signal (standard signal) make it easier to use with other devices that are manufactured under common standard temperature transmitter, level transmitter Tx and a symbol of transmitter.
Transmitter functions are as follows.
Ø Converse change variables to measure the desired change, such as flow rate and differential pressure.
Ø Amplifier serves to amplify the signal changed to a larger size. To make it easier to transport and to reduce the problem of signal loss or noise interference.
Ø Standard signal of the amplifier must be expanded in scale.
4-20 mA, 1-5 V, 3-15 psi.
Ø Transmit a signal to the standard. The measured signals can be exported in various formats as follows.
- Electrical Signals into an electrical signal in the form of electricity. Or voltage (4-20 mA, 1-5 V).
- Pneumatic Signals are signs of pressure (3-15 psi).
- Hydraulic Signals is a sign of fluid pressure (3-15 psi).
- Telemetered Signals in the form of radio frequency signals.
1.16 Transducer is an electrical device for the measurement of Sensor and then sends an electrical signal to the control room. The transmitted signal will not be in the form of Standard Signal Transmitter, such as Vibration Transducer, which may be -5 to +5 V or 1 to 10 V was.
1.17 Value Sizing is a small selection of other components of Value types are associated with the system to work properly.
1.18 Set Point is the goal of control, such as Steam Temp. Set Point = 560oC and the control variables we measured, that is, Measured Variable Controlled Variable and generally will be the same. The temperature of the water and Measure Variable Controlled Variable temperature.
In some cases, control the water level in the tank Controlled Variable is the water level in the Measured Variable can be measured in terms of pressure or pressure difference can then convert the pressure into the water or the Flow Rate. is the Measured Variable Differential Pressure.
1.19 Bench Set is a test of the movement of Actuator Spring Range by the wind into the Diaphragm Chamber for Actuator a move that is not connected to the Value Plug a test of the diaphragm that works according to its pressure. scheduled or not. This is a preliminary indication that the diaphragm has a problem like a leak or a tear or other defect to be corrected before there is a problem in actual use.
1.20 Diaphragm Pressure Span is the difference between maximum and minimum values, such as Diaphragm Pressure Range 40 to 400 kPa at the Span is 400 - 40 = 360 kPa.
1.21 Single Acting Actuator is an input of energy Actuator (Pneumatic, Hydraulic) in one direction will cause the spring to move in a direction opposite to the force of the pneumatic or hydraulic.
The Control valve handbook, third edition, FISHER Controls international LLC, USA.
1.22 Double Acting Actuator is to have the input of energy Actuator (Pneumatic, Hydraulic) in the second direction is not spring. Movement in both directions must use the force of Pneumatic or Hydraulic) or the need for external power in both directions.
1.23 Dynamic Unbalance is the presence of the load. One caused by the fluid force acting on the Value Plug in an open Value.
The technical documents, training of Control Valve, Pisanu course Mr. Brown doing the Navy F: I, Division of Training Equipment, Training Department, Electricity Generating Authority of Thailand.
1.24 Fail Closed as the conditions that apply to certain systems where security is a major concern. When no power source (Pneumatic, Hydraulic) Value will be placed in the closed position. Usually closed by spring force.
1.25 Fail Open Fail Closed, but Value is the same as when the Power Source Fail.
1.26 Fail Safe is to determine the position of the Value that will be placed in any position on the Power Source Fail is perhaps the open end (fully open) closed with (fully closed) or open the original (lock up), depending on. Control Regulatory controls or conditions with regard to the safety of the system as a safety valve should be fail safe, it is fully open.
1.27 Flow Characteristic is the relationship between Flow Rate through the Open Value and Percent of Value in mind that there are two general types of 0-100%.
- Inherent Flow Characteristic.
- Installed Flow Characteristic.
- Inherent Flow Characteristic. Is the relationship of the Flow Rate to Value with the movement of Stem Travel, or%, to close the opening of the Value of the closed end to the Rated Flow with a Pressure Drop for drop box service valve at one value (Constant DP Across. Value).
- Installed Flow Characteristic. Is the relationship of the Value Flow Rate to Stem Travel or% off of the Value of the closed end to the Rated Flow with a Pressure Drop for drop box service valve is changed in accordance with the flow and condition of the Process. this value is the actual time used.
1.28 Flow coefficient (Cv) is a constant that depends on the geometrical shape of the object, the fluid flow. This is the Flow Capacity to the Cv is determined at 60oF, Pressure Drop 1 psi Measured Flow Rate of Water-US Gallon / Minute Flow Value for Cv are the specific Value, or a device such as Cv. Orifice plate or the Control Value table Cv Cv values to be computed at a flow rate.
The Control valve handbook, third edition, FISHER Controls international LLC, USA.
1.29 Normally Closed Value is defined as the Fail Closed.
1.30 Normally Open Value is defined as the Fail Open.
1.31 Push Down to Close Construction of Value Actuator is Direct Actuator (direct acting) is when there is pressure at the Diaphragm Chamber will make Value Stem cell into the Value disable this behavior as a Value over, such as Value faucet. rotate the hand lever valve is rotated to move the valve to close.
The Control valve handbook, third edition, FISHER Controls international LLC, USA.
Push Down to Open Construction of Value Actuator Direct Actuator is a part of the Reverse Value Plug it into a pressure inwards on the Diaphragm Chamber will move to the Value Value Stem to Fluid flow through.
The Control valve handbook, third edition, FISHER Controls international LLC, USA.
1.32 Rangeability is the Maximum Range / Minimum Range for 100 / 1 means that the Control Value is the ability to control the flow well even Flow Rate up to 100 times that of the Minimum Controllable Flow Rate is normally controlled Flow Rate 1. m3/hr to 100 m3/hr, but may be used by the Capacity, Pressure Drop Performance remains the same, or, most often used with the Rangeability Instrumentation and Instrument or device is called a squeeze - to span the pressure. transmitter with a rangeability 50 / 1 means that one can calibrate the measurement range 0 -1 bar or 0 - 50 bar or in the case of a compressed span of the error or accuracy, they also must consider that. in a neighborhood that is acceptable or not.
1.33 Rated Flow Coefficient Cv is located in the Value Rated Flow Rate.
1.34 Rated Travel Travel is the movement from the closed position to a position or positions on the highest Rated Full Open Flow Rate requirements.
1.35 Seat Leakage of the leak of Fluid through Value when in the closed position in the state of Temperature and Differential Pressure required in practice is not able to make the valve blocking the flow of 100% will have leakage values that are in charge. acceptable to the table.
The Control valve handbook, third edition, FISHER Controls international LLC, USA.
1.36 Vena Contracta the position of the Maximum Fluid Velocity and Minimum Pressure.
The Control valve handbook, third edition, FISHER Controls international LLC, USA.
1.37 Calibration Curve for the Relationship between Input and Output data.
The Control valve handbook, third edition, FISHER Controls international LLC, USA.
1.38 Calibration Cycle.
Is to record the relationship of the Input and Output from 0 to 100% and from 0% to 100 back to the basics for the analysis of problems in the future. Such as checking the drift occurs.
The Control valve handbook, third edition, FISHER Controls international LLC, USA.
1.39 Hunting is the state of the Oscillation of the control signal to the Final Element can not get to the Set Point in the Control Value Most of Control Signal from the Positioner, which is not stable (unstable signal) can not control the PV was based. they need. These symptoms may be caused by interference. Radio transmission time. Or from devices that generate high-power radio service. This will make the stem valve may be moved at any time. In order to run toward the set point, but it can not be approached. It was moving back and forth all the time. This results in a direct bearing on the mechanical equipment, including packing, positioner mechanism, actuator, etc., that will wear faster than normal as well as the process deviation is the deviation of the production or quality of products as was the case with fuel oil. flow control valve or valves to control flow rate of fuel used in combustion. It makes the combustion temperature is not constant. If steam is produced, it will be the pressure and temperature of the steam has not been affecting the functioning of the device makes the system more difficult as a result.
1.40 Instrument Pressure Output Pressure of the Controller (I / P) to the order of the Action to Control Value.
Show the location of the instrument pressure.
The Control valve handbook, 3rd edition, FISHER Controls international LLC, USA.
1.41 Loading Pressure is the pressure of the Pneumatic Actuator works if no I / P converter for Instrument Pressure Loading Pressure can be used instead.
1.42 Operating Media is a Fluid Flow Rate controlled by the Control Value Max. Flow Rate 100 m3/hr at 100.
1.43 Range is the range of measurement applications or areas such as 3 to 15 psi, -40 oC to 100 oC and so on.
1.44 Span Range is the range of 3 to 15 psi as the Span is 15-3 = 12 psi and so on.
1.45 Repeatibility. The ability of the Instrument, which can be measured the same value or close to the original value under any condition was based on the measurement method, the measurements, the device used to measure (quality), the location of the temple, Condition. (such as temperature, pressure, etc.) is usually measured in a short time he tells the Standard Deviation is a measure of the pressure of the water temperature is 80 oC, Flow Rate 50 m3/hr assumed that the P value was equal to 10.0. later time T, and the bar Flow Rate changes until it returns to 80 oC and 50 m3/hr 10.0 bar P value should be measured by the same means that every time you go through the Process to the measured value. With time, it should be the same.
The Control valve handbook, 3rd edition, FISHER Controls international LLC, USA.
Not a measure of P = 10.0 bar.
The second measure is P = 11.5 bar.
The third measure P = 9.0 bar.
The conditions were at that time (the measurement) is the same as the temperature is 80 oC, Flow Rate 50 m3/hr and is called there. Repeatibility bad.
The Repeatibility. On the other hand, is meaningful. The ability of the Instrument to Measure and Output out by the Input entry is maintained under Operating Condition was in a short time during one such Multimeter Measurement Voltage supply voltage (Ideal Power Supply) is required. measured under the conditions of the original value was measured as 10.0 VDC for 10 times, etc. In a short time.
1.46 Reproducibility. Instrument is capable of measuring the value and the value of Input Output was originally entered. Under Operating Condition, but time is a measurement in the range 1 month 3 months or 1 year, this measure was 100 bar for 3 months, it should be measured at 100 bar with the Operating Condition the Instrument to be. Reproducibility is good too.
1.47 Drift has a similar meaning. Reproducibility. Or may be called as the same words that should not cause any Instrument Drift in a short period of time or for a few months, the Drift has two types.
- Point Drift by the original Operating Condition of the Output is measured as much by the change from the original value at the Input Signal.
- Calibration Drift is the introduction of the new Calibrate Instrument was made by comparing the results obtained with the Calibration Report or Calibration Curve was that there was much difference in any of the Calibrate settings such as
The first was to calibrate.
Input (%).
Output (volt).
0.0.
5.0.
25.0.
10.0.
50.0.
15.0.
75.0.
20.0.
100.0.
25.0.
The new value through the use of a 1-year ago.
Input (%).
Output (volt).
0.0.
4.5.
25.0.
10.4.
50.0.
14.5.
75.0.
19.5.
100.0.
24.3.
Drift of the sample that have occurred about 0.5 in every instance by the Drift of the device to tell the figures are the% Output Span drift = 0.5x100 / 20 = 2.5% of output span.
1.48 Static Error Error is measured at the same conditions. But the difference is negligible.
1.49 True Value (the value that is true, true).
True Value = Instrument Readout (reading from a meter) - Static Error.
1.50 Sensitivity is the ratio between the Change of Output / Change of Input under Steady State Condition.
1.51 Dynamic Characteristic occurrence of three types.
- Step Change Input changes immediately.
- Linear Change Input value changes in a gradual increase or decrease, also known as the ramp signal.
- Sinusoidal Input value changes in a Sine Wave.
Change the nature of these causes Dynamic Error, Lag, Speed of Response, Fidelity, Etc.
1.52 Dynamic Error Error is caused by the condition of the Dynamic Characteristic and 3.
1.53 Lag or time lag when the input changes the nature of the device. Will not be able to respond immediately to the Time Lag is always
1.54 Speed of Response is the ability of the Instrument in response to changes in the Input Speed is an extremely good that can detect very early changes in the Input.
1.55 Precision Error Error is from the Conformity of a measuring device such as R = 1.592 Mega Ohm Resistor with Error 154 Ohm Multimeter is used to measure and read by anyone. (Assuming that there is no Error that reads values), the value of boost pressure. Read it at about 1.5 Mega Ohm with the actual value is at 1.592 +154 Mega Ohm reading of Scale, called the Precision Error Error is caused by the limits of the measuring Scale Reading.
1.56 Zero Error of Measurement Error in a lower range Input is configured as a% Of Ideal Span.
1.57 Zero Offset is defined as normal, with no Input Indicator Instrument is to be displayed at the center. If the reading is not zero, that is the Zero Offset to adjust or Calibrate the Indicator shows a value center, and to save them if the Zero does not really illustrate clearly the case of the Pressure. Gauge is placed in a normal atmosphere. Pointer Indicator pointer value, or should the value "0".
Mechanical Hystersis 1.58 Backlash is the most happening with Gear, Linkage, Mechanical Transmission Device as well as figure out the gear. When rotated clockwise. The position of the gear position to another. However, when rotated counter-clockwise. Position of the gear transmission is not in its original position. Backlash is a little change should be minimal. However, this does not eliminate it. Due to the limitations of the mechanical parts that need to be involved Clearance. The device is a mechanism or a Non-Rigid Mechanism.
2. Introduction to Control Value.
The control system is a device that serves to drive the Final Element of the control signals from the Controller and served in the refinement or Manipulated Variable Process below.
Final Element or final control element is composed of many types of equipment.
- Control Value (Pneumatic / Hydraulic).
- Motor Operated Value.
- Pneumatic / Hydraulic Damper.
- Pump (Metering Pump).
- Pneumatic / Hydraulic Cylinder.
- Shut off Value (pneumatic / Hydraulic).
- Linear Drive Actuator.
- Other Mechanical Devices.
It can be seen that the control valve is part of the final element is an important and best in the world.
2.1 of the Control Value.
Final Element Control Value is one of the main functions is important. Control the rate of fluid flow (fluid flow control) is discussed in a Control System that serves to adjust the Manipulated Variable to get the value of Set Point by checking with the Instrument that monitors a variable or Measured Variable (. Controlled Variable) or PV and then send a signal to the Controller to compare values with the Set Point (SP) If the value of Offset or Error (SP-PV) occurs in the Controller, it will add to the Control Value to adjust. Process until the desired value or Error = 0 to end processes.
2.2 Types of Control Value.
By following the motion of the Drive Shaft.
A. Linear Shaft Value or Sliding Stem Valve Drive Shaft is a movement in a straight line as shown.
The Control valve handbook, third edition, FISHER Controls international LLC, USA.
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